In a known form of apparatus for controlling the weight of a hollow member which is produced from for example thermoplastic synthetic resin by an extrusion blowing process, a preform, for example of a tubular configuration, is continuously extruded from an extruded head connected to the output of a screw press, the preform being extruded through a slot-like discharge opening in the extrusion head. After the preform has been blown in a blow mold into the required form of hollow mmeber, the weight of the hollow member is established and compared to a desired or reference value, and the width of the discharge opening of the extrusion head is adjusted in dependence on the result of the comparison operation, in order to regulate the volume of the material used to form the preform.
When producing preforms which are of a tubular configuration, the opening in the extrusion head is generally in the form of an annular slot. As this is the situation which arises most frequently in practice, it will be assumed in the specification hereinafter, for the sake of simplicity, that the preform to be produced is of a generally tubular configuration, and that the discharge opening of the extrusion head is thus in the form of an annular slot. It will be appreciated however that the present invention may also be used in relation to preforms which are of a different configuration, for example which are of a film or foil nature.
For the purposes of producing a satisfactory final product, for example a bottle, canister or the like, the preform which must be of a minimum length must comprise a certain minimum amount of material which must also be properly distributed over the length of the preform, so as to have a uniform and correct wall thickness. In this respect, the minimum amount of material which is required to make a satisfactory product should not be exceeded, or should not be exceeded to a substantial degree, as otherwise a greater amount of material than is necessary from the technical point of view would then be consumed. It will of course be appreciated that in general the preform will always be longer than the corresponding length of the blow mold cavity, the excess material being squeezed out by the blow mold and/or the means for feeding the blowing pressure agent, for example a blowing mandrel. However, the waste or excess portion can generally be recycled. On the other hand, an excess of material which is due to the preform being of excessive wall thickness cannot be recuperated as the excessive wall thickness is still to be found in the hollow member so that the wall thickness of the hollow member is greater than necessary. In other words, economical production requires that the wall thickness and thus the weight of the preform must be properly influenced and controlled, as correct wall thickness and thus correct weight and correct volume are necessary conditions if the hollow member, as the final product of the process, is not to include a greater amount of material than the amount strictly required to perform its technical function.
When the article is produced by means of an arrangement including an extrusion head with a storage chamber from which the molding material can be emptied by means of a piston, the requirement for establishing a given length of material for each preform, being the same length from one working cycle to the next, does not give rise to any difficulty as the amount of material used in each preform is precisely established by virtue of the volume of the storage chamber remaining unaltered. In that case it is only necessary for the width of the slot forming the discharge opening of the extrusion head to be set in such a way that the given amount of material is so distributed thereover as to ensure that the preform has the minimum amount of material required to produce a satisfactory final product.
When however continuously extruding a preform, there is no possibility of precisely determining the amount of material in each preform, by way of an interposed storage chamber in the extrusion head, as just referred to above.
One form of apparatus which has been disclosed for controlling the weight of a hollow member produced from thermoplastic material by an extrusion blowing process, of the general kind outlined above has not fulfilled the expectations placed on it. This is essentially because, although the entire opening procedure and movements of the blowing mold and the parts thereof are rigidly controlled in respect of time, and the speed of rotation of the screw of the press is constant, fluctuations can arise in regard to the conveying output in each unit of time and thus also the speed of preform discharge. This is essentially because there are operating in the extruder many other influencing parameters which also influence each other. These parameters are for example the apparent or bulk density, temperature, viscosity and swelling characteristics of the material. The preforms are thus of different lengths and possibly weights, with the result that the preforms tend to suffer from elongation phenomena which vary in dependence on the weight of the preforms; such phenomena have the result that, even when the overall weight of the finished hollow member corresponds to the reference or desired weight, the distribution of material within the hollow member may be so uneven or irregular that the end product is useless. This for example is to be attributed to the fact that, in spite of the preform being of the proper wall thickness, the amount and thus weight of material which issues from the discharge slot of the extrusion head in each unit of time fluctuates and consequently the respective sections of the preforms which are to be accommodated in the mold cavity undergo varying degrees of extension. In this case, the preform would be found to be of excessively low or excessively high weight, even though the width of the discharge slot was correctly set. A further disadvantage of such apparatus is that, when using a time-dependently programmed control for the distribution of wall thickness of the preform over the length thereof, the individual regions of the preform do not always occupy the proper position relative to the mold. In this case, the apparatus cannot achieve useful control in respect of the weight of the end product.